The invention relates to a molding element for molding a cutout in a tread of a tire, and to a mold equipped with at least one such molding element.
Patent FR 2 759 323 describes a novel tread pattern for a tire tread which is particularly advantageous as far as the traveling noise and wear performance is concerned. In this application, there is proposed a tread pattern for a tire comprising a plurality of cutouts, the walls of which are connected by at least one connecting bridge; there is also proposed a production method according to which an insert, for example made of paper, provided with at least one orifice allowing the rubber mix to pass during molding in order to produce a connecting bridge, is introduced into a tread before molding and vulcanization.
In another patent FR 2 759 321, another process is proposed for producing such a tread pattern, using a mold comprising an upper part and a lower part, each of said parts being provided with molding elements which cooperate to mold cutouts provided with connecting bridges. This method, although very advantageous, however, requires, first of all, a tread to be manufactured and then, secondly, said tread to be assembled on a tire blank devoid of a tread.
One object of the present invention is to produce a tread comprising at least one cutout provided with at least one connecting bridge by molding the tread directly during the molding and vulcanization of a tire provided with this tread without having to introduce an inserted material into a tread.
To achieve this object, there is proposed a molding element intended to be fitted on a mold for molding a cutout in a tread of rubber mix, said cutout having its main walls connected by at least one connecting bridge of rubber mix. Each mold part has a molding face for molding the running surface of said tread, and can be moved in a direction of molding/demolding.
The molding element according to the invention comprises an anchoring part intended to be fastened to a mold part and prolonged by a molding part intended to project from the molding face of said part. The molding part of the molding element according to the invention comprises at least two extensions forming prolongations and the faces of which are intended to mold walls defining the cutout, at least one of these extensions comprising at least one arm which is mounted to pivot about a pivot pin fastened to said extension.
xe2x80x9cCutoutxe2x80x9d is understood to mean, in the field of tires for passenger vehicles or heavy vehicles, either a groove, characterized by a width at least equal to 3 mm, or an incision, characterized by a width of less than 3 mm.
In the molding configuration, an arm which is mobile in rotation is said to bear on another extension of the same molding element when part of one of its wall faces is in contact with said other extension. Within the scope of the invention, it may be envisaged that a mobile arm mounted on a extension is in contact with a mobile arm of another extension of the same molding element.
At the start of the demolding operation, after molding and vulcanization of a tread with a mold fitted with a molding element according to the invention, each arm which is mobile in rotation is subjected to the stresses exerted by the connecting bridges molded in the orifices and will then turn to permit extraction of the molding element from the tread without breaking the molded connecting bridges. Of course, this demolding is only possible owing to the mechanical properties of the rubber mixes (in particular, the possibility of great deformation without there being any breaking of said mixes).
Advantageously, elastic return means may be provided to force each arm which is mobile in rotation to resume the position which it occupies in the molding position, so as to ensure good molding geometry. With the same aim, alone or in combination with the previous arrangement, means for retaining said mobile arms in the molding position may be provided, these means being effective up to a predetermined level of stress (which is of course less than the stresses exerted by the mix during demolding in order to permit demolding).
The invention also relates to a part of a mold for a tire tread. This part is provided with a molding face for molding the running surface of the tread, and may be displaced in a direction of molding/demolding. This mold part comprises at least two elements in relief projecting from the molding face of said part, at least one of said elements in relief comprising a to support bearing at least one arm mounted to pivot about a pivot pin attached to said support.
The mold part is characterized in that, in the molding configuration, at least one arm mounted to pivot on an element in relief bears on another element in relief to form at least one orifice intended for rubber mix to pass through during the molding of a tread.
xe2x80x9cBearing of a pivoting arm on an element in reliefxe2x80x9d is understood to mean that this arm, in the molding configuration, has a face which is in contact at least in part with at least one face of said element in relief.
The mold part according to the invention makes it possible, in particular, to mold in a tread a cutout, the main walls of which are connected by one or more connecting bridges, at least one of said bridges being located entirely beneath the running surface of the molded tread (for this, at least one mobile arm is at a distance strictly greater than zero from the molding face of the mold part).
After molding and vulcanization of a tread and at the time of demolding (that is to say, extraction of the elements in relief from the tread), the arms, which are mounted to pivot in a plane containing the direction of molding/demolding, will pivot under the action of the forces exerted by the rubber mix passing through the orifices of the element in relief. Once the mobile arms have pivoted, it is then possible to disengage the elements in relief from the tread without any breaking of the bridges.
Preferably, elastic return means may be provided to force the pivoting arms to resume the position which they occupy in the molding position. Furthermore, means for retaining said mobile arms in the molding position may be added, these means being effective up to a predetermined level of stress (which is of course less than the stresses exerted by the mix during demolding).
The invention which has been set forth above will be better understood with reference to the following description, given with respect to the drawings, which show several variants according to the invention which are shown in non-limitative manner: